Generally, in a spinning mill, it is common practice to monitor variations of the thickness of a spun yarn that appear as thicker and thinner portions of the yarn. If a defect is found, the yarn is cut to remove the defect. A testing device is employed for this monitoring operation. This testing device is equipped with measuring capacitors consisting of a pair of electrode plates disposed across a yarn sample passage. If the weight of the traveling yarn sample varies, and if the dielectric constant is thereby changed, the electrical capacitance across the measuring capacitors is varied. The resulting electrical capacitance variation is electronically detected. Thus, the yarn evenness or unevenness is evaluated.
It is known that yarn unevenness including the aforementioned defect is caused by mechanical defects in the spinning process, i.e., improper adjustment or defects in mechanical components. Accordingly, an experienced technician usually has analyzed the circumstances of periodic defects contained in yarn thickness variations represented by the output signal from the testing device to detect the presence or absence of any mechanical defect during a spinning process.